The present invention relates to an integrated device for shielding the injection of charges into the substrate.
As is known, when driving inductive and/or capacitive loads, for the correct behavior of the system it is necessary to recirculate or discharge the current accumulated in the load towards the ground. If a load is driven by an integrated device, the substrate of the device is clamped to the ground and the current recirculation occurs through the epitaxial pocket diode (which constitutes the cathode) and the substrate (which defines the anode of the diode). In order to recirculate or discharge the current stored by the load at the switch off of the load driving devices, this diode is forward biased and the epitaxial pocket reaches a negative voltage with respect to the ground. In this condition the epitaxial pocket with negative voltage (recirculation or flyback pocket) injects current into the substrate, behaving like the emitter of a parasite NPN transistor, and any other epitaxial pocket of the same device, which is at higher voltage than the substrate, collects the injected electrons, behaving like the collector of said parasite transistor.
This injection of charges into epitaxial pockets at high voltage must be avoided, as it entails the following problems:
(1) the circuit has an undesirable dissipation of power;
(2) if the epitaxial pockets are the base of a lateral PNP transistor, this injected charge causes said PNP transistor (e.g. the driving ones) to switch on again;
(3) the base current of lateral PNP transistors increases, with malfunction of the device.
In order to avoid the above described problems, two solutions are currently used, both having the purpose of creating a path with lower impedance than the other epitaxial pockets of the circuit and therefore such as to collect all the current injected by the epitaxial pocket with negative voltage with respect to the ground. Such solutions have a similar structure (see FIG. 1), comprising an isolated epitaxial pocket surrounding the entire epitaxial pocket which can reach a negative voltage with respect to the ground. Said isolated pocket comprises, in its interior, a buried layer and a sinker region, both being heavier doped than the epitaxial layer. This structure is then connected to the ground or to the supply, depending on the adopted solution.
Such known solutions, however, do not solve the problem of injected charges. In fact on one hand they are unable to completely eliminate the current towards the circuit and on the other they do not reduce the dissipated power. The solution comprising the connection to the ground is furthermore scarcely competitive with respect to the epitaxial pockets at high voltage which tend to better gather the injected charges than the epitaxial pocket connected to the ground, while the solution teaching the connection to the supply is disadvantageous in terms of power dissipation since the injected current is collected towards the highest voltage point.
Another known solution is described in the U.S. patent application No. 07/099.044 of 9.21.1987 (now U.S. Pat. No. 4,890,149) assigned to the same Assignee and comprises four integrated structures constituting charge collector regions or structures adapted to limit the voltages of the regions of the device. Though this solution fully solves the problem of injected charges, it is however not always applicable due to its considerable bulk.